2,5-Dihydroxybenzenesulfonic acid derivatives and specifically calcium dobesilate, ethamsylate and persilate are known in the art as active agents for the treatment of male sexual dysfunction and other vascular disorders of endothelial origin, both alone and in combination with other agents. U.S. Pat. No. 6,147,112 describes a method for the use of 2,5-dihydroxybenzenesulfonic acid derivatives, preferably calcium dobesilate, ethamsylate and persilate.
Calcium dobesilate or hydroquinone calcium sulfonate, with the chemical name 2,5-dihydroxybenzenesulfonic acid calcium salt, is being sold as Dexium (Delalande) and Doxium (Carrion), and a process for its preparation is described in U.S. Pat. No. 3,509,207.
The potassium salt of 2,5-dihydroxybenzenesulfonic acid, formula (Ia), is also an important compound.

Combinations of active substances comprising a 2,5-dihydroxybenzenesulfonic acid compound including potassium 2,5-dihydroxybenzenesulfonate are disclosed in International Application Publication No. WO2005/013962. Compositions and methods of use for 2,5-dihydroxybenzenesulfonic acid compounds and their pharmaceutically acceptable salts, including potassium 2,5-dihydroxybenzenesulfonate, are discussed in U.S. Patent Publication No. 20070149618, which provides methods for (a) treating skin cancer; (b) treating cancer of the organs; (c) treating leukemia; (d) improving the efficacy of chemotherapy, radiation therapy and/or cancer immunotherapy; (e) treating rosacea; and (f) treating psoriasis by administration of a composition comprising at least one 2,5-dihydroxybenzene sulfonic acid compound or a pharmaceutically acceptable salt thereof, and, optionally at least one therapeutic agent.
In view of the importance of the potassium salt of 2,5-dihydroxybenzenesulfonic acid, it is desirable to develop a process for its manufacture by a simple method with improved yield, purity and stability.